An SBS-PCM reflects a phase conjugation wave to compensate a distortion of a laser beam, generated during a laser amplification process, so that it can be easily applied to a beam splitting high-power laser. The beam splitting amplification laser using the SBS-PCM is disclosed in U.S. Pat. No. 5,832,020A and Korean Patent No. 1003185200000.
FIG. 1 is a schematic diagram of a beam splitting amplification laser. which is disclosed in detail in Korean Patent Application No. 10-1999-00011876. The prior art beam splitting amplification laser shown in FIG. 1 provides a light interrupter that prevents a part of a laser beam that is emitted from a laser beam generator and passed through optical elements of a system from inputting to the laser beam generator again to damage the laser beam generator. FIG. 1 is a schematic diagram for explaining splitting and amplification of a laser beam. Referring to FIG. 1, a laser beam emitted from a laser source 5 is amplified through beam amplification stages 10, 11, 12, . . . . The amplified laser beam 13 is provided to a system that requires the laser beam.
If a part of the laser beam is reflected and input to the laser source 5 again, the laser source 5 is damaged. To prevent this, the prior art arranges a ¼ phase plate 6, an SBS-PCM 7 and an optical amplifier 8 and uses polarization to block the beam input to the light source.
In the above-described beam splitting amplifier, laser beams split by each beam splitter are reflected from each SBS-PCM and recombined if required. If the recombined laser beams have a phase difference of 180 degrees therebetween, the central intensity in the spatial distribution of the recombined laser beams becomes “0” to deteriorate the spatial distribution of the laser beams. Thus, a high-quality laser beam cannot be obtained.
The laser beams reflected from the phase conjugation mirrors have different phases because the stimulated Brillouin scattering occurs at different positions of the phase conjugation mirrors. Accordingly, laser beams reflected from different phase conjugation mirrors have different phases and, when these laser beams are recombined, the central intensity of the recombined beams is different from the peripheral intensity. To obtain a high-quality laser beam having a uniform spatial distribution of a combined laser beam, it is required to control phases of laser beams reflected according to the stimulated Brillouin scattering to make relative phases of the laser beams “0”.
To solve the aforementioned problem, studies have been carried out in various ways.
FIG. 2 is a schematic diagram showing a method of controlling phases of laser beams according to focus overlapping using an SBS-PCM to lock the phases. This method focuses a plurality of laser beams 21, 22 and 23 on a single SBS-PCM 20 through a lens 24 so as to superpose the multiple laser beams on one focus. In this case, the multiple laser beams are reflected from the same position according to scattering so that they have the same phase. However, this method has the following problems.
First of all, since the multiple laser beams must be focused on a single phase conjugation mirror in order to amplify the laser beams, a stimulated Brillouin scattering medium loses its function or presents non-linearity in the case of a high incident energy. This remarkably reduces reflectivity and phase conjugation degree. A beam recombination amplification system is used to obtain a high-energy laser beam. The above-described focus overlapping phase locking method is difficult to use when the incident energy exceeds the energy that the medium of the phase conjugate mirror can stably reflect. Furthermore, the laser beams should be arranged such that their focuses are overlapped on one phase conjugate mirror. Thus, as the number of laser beams increases, the laser beams become difficult to arrange. Moreover, the number of the laser beams is spatially restricted. Accordingly, an output energy is also restricted so that a high-power energy cannot be obtained.
FIG. 3 is another method for making a phase difference between split laser beams “0”. This method is a phase locking technique according to stokes wave back seeding using a conventional SBS-PCM. Here, the stokes wave means a laser beam having the same frequency as that of a reflected wave that is reflected according to the stimulated Brillouin scattering. This technique inputs a laser beam to be reflected to the front side of a phase conjugate mirror and, simultaneously, seeds a laser beam at the back of the phase conjugate mirror. In this case, a stokes wave input to the back side of the phase conjugate mirror is amplified so that the effect as if the incident beam is reflected is obtained.
In FIG. 3, when an incident laser beam 38 passes through a polarization beam splitter 36 and a Faraday rotator 34, polarization of the incident laser beam is rotated by 45 degrees. Then, when the laser beam is reflected from a phase conjugate mirror 30 and passed through the Faraday rotator 34 again, its polarization is rotated by 45 degrees again. As a result, the reflected laser beam 32 has polarization that is rotated by 90 degrees with respect to the polarization of the incident laser beam 38. A laser beam 40 reflected by the polarization beam splitter 36 is identical to the laser beam reflected by the phase conjugate mirror 30 so that the laser beam 40 becomes a stokes wave having a frequency lower than that of the incident laser beam 38 and it can be used as a seeding beam. When laser beams 45, 45′ and 45″ to be reflected are respectively input to the front sides of the phase conjugate mirrors 43, 43′ and 43″ and, simultaneously, the seeding beam 40 is split into beams 42, 42′ and 42″ using beam splitters 41, 41′ and 41″ and input to the back sides of the phase conjugate mirrors 43, 43′ and 43″, respectively, the seeding beams 42, 42′ and 42″ can be used for phase-locking the amplified laser beams 45, 45′ and 45″. Here, it is possible to make a phase difference among the seeding beams 42, 42′ and 42″ become “0” in order to make a phase difference among the reflected laser beams 45, 45′ and 45″ “0” by controlling the positions of the beam splitters 41, 41 ′ and 41″.
When the same stokes wave is seeded in the stimulated Brillouin scattering medium, the same stokes wave is amplified so that the phases of the beams reflected from the SBS-PCMs are locked. However, the phase locking technique according to the stokes wave back seeding has the following problem. First of all, a structural arrangement of a system for carrying out the technique is complicated because the back seeding stokes wave should be generated and passed through focuses. That is, beams reflected from the SBS-PCMs should be back-seeded and, when there are many split beams, all of the split beams require back seeding. To avoid this, only a part of the incident beam is back-seeded. In this case, however, the incident beam must have a wide frequency band and include the frequency of the stokes wave. However, this may deteriorate the quality and performance of reflected beams when the fact that the stimulated Brillouin scattering occurs well when a frequency width is small is considered. Even in this case, the structural arrangement becomes complicated and difficult as the number of split beams increases.